The catecholamine neuromodulators dopamine and norepinephrine play key roles in cognition and psychiatric pathophysiology. Much of the evidence for this comes from human studies using positron emission tomography (PET), which allows direct, in vivo measurement of aspects of catecholamine function, such as synthesis or transmission. However, the invasiveness and high degree of specialization associated with PET limit its applicability for clinical use and research, especially in pediatric populations and repeated, longitudinal assessments of illness course and treatment response. This underscores the need for biomarkers that, like PET-based measures, reflect neurobiological processes relevant to illnesses and treatments but are more scalable and easily acquired.